In the production technology for silver halide photographic emulsions, there is a need to increase the sensitivity of photographic emulsions. Chemical sensitization and spectral sensitization are known as useful methods for increasing the sensitivity of silver halide photographic emulsions.
Spectral sensitization is a technique in which the sensitivity of a photographic emulsion is increased by including a sensitizing dye in the silver halide photographic emulsion and thereby expanding the sensitive wavelength region of the silver halide photographic emulsion, which had been limited to the shorter wavelength region of visible light, into longer wavelength regions. Cyanine dyes and the like are the principal sensitizing dyes, though many other sensitizing dyes and methods for using these are also known.
In general, spectrally sensitizing dyes are added to an emulsion which has undergone chemical ripening prior to coating. Methods in which they are added to the emulsion prior to the start of or during chemical ripening are disclosed, for example, in U.S. Pat. No. 4,425,426. Furthermore, methods in which the spectral sensitizing dye is added to the emulsion before the formation of the silver halide grains has been completed are disclosed in U.S. Pat. Nos. 2,735,766, 3,628,960, 4,183,756 and 4,225,666.
In particular, U.S. Pat. Nos. 4,183,756 and 4,225,666 disclose that adding the spectral sensitizing dye to the emulsion after the formation of a stable nucleus during silver halide grain formation gives advantages such as an increase in the photographic speed and a strengthening in the adsorption of the spectral sensitizing dye by the silver halide grains. Furthermore, JP-A-61-133941 and JP-A-61-160739 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") disclose a method of spectral sensitization in that the chemical sensitization is carried out in the presence of a spectral sensitizing dye.
Thus, the methods of adding spectral sensitizing dyes have diversified in recent years and have come to show such features as a strengthening in the adsorption of the sensitizing dye by the silver halide grains when compared with conventional addition methods and the chemical sensitization site has been focused (i.e., the chemical sensitization site is carried out at only the specific place).
It is well known that heterocyclic compounds having mercapto groups are effective as antifoggants for photographic emulsions, while it is also known that compounds such as these reduce the sensitivity of a photographic emulsion. This is described on pages 344 to 346 of Volume 3 of The Theory of the Photographic Process edited by C. E. K. Mees and T. H. James.
On the other hand, JP-A-51-77224 discloses that if a heterocyclic compound having a mercapto group is included together with a trimethine cyanine sensitizing dye in the silver halide emulsion after chemical sensitization has been completed, the spectral sensitization action of the cyanine dye is strengthened. Moreover, JP-A-51-36130 discloses that heterocyclic compounds having mercapto groups increase the intrinsic sensitivity of silver halide emulsions in which the majority of the silver halide is composed of either silver bromide or silver chloride and in which the silver halide grains have a cubic crystal form.
JP-A-49-64419 discloses that the spectral sensitization action of a cyanine dye having a pyridine nucleus in at least one of the two hetero nuclei, in other words a pyridinocyanine dye (or a pyridocyanine dye), is increased by using, in combination with the dye, certain types of mercapto compounds having acid groups.
As described above, there have been disclosures of several spectral sensitization methods such as the method of adding the spectral sensitizing dye to an emulsion or combining it with a heterocyclic compound having a mercapto group. But there remain many points for improvement in the strength of adsorption of the sensitizing dye by the silver halide grains and for improvement in chemical sensitization and/or high efficiency of the spectral sensitization.
It is known that some cyanine dyes form J-aggregates when they are added to silver halide emulsions and display an absorption band known as a J-band. The features of the J-band, which is contrasted with the M-band produced by the dye monomer, are that the .lambda.max, the maximum absorption wavelength, is longer than that for the M-band, it is sharper (the half-value width is narrower). These features are indispensable for providing the color sensitivity for a color photographic material. There is no means for directly measuring the size of a J-aggregate but, by reference to theory, A. E. Rosenoff, K. S. Norland, A. E. Ames, V. K. Walworth, G. R. Bird et al. have indicated in Photogr. Sci. Eng., Vol. 12, No. 4, p. 185 (1968), it is possible to estimate that the J-aggregate is larger the longer the wavelength of the .lambda.max of the J-band or the narrower the half-value width.
As a result of diligent study into methods of spectral sensitization, the present inventors have discovered that by using cyanine dyes which form a J-band when added to a silver halide emulsion and carrying out ripening by adding these dyes to the emulsion under conditions such that the relative quantum yield .phi..sub.r of spectral sensitization assumes a value for .phi..sub.r in the emulsion which is 1/2 or less and more preferably 1/3 or less of the value obtained when the ripening conditions from the time when the dye is added to the same emulsion until the time of coating are 40.degree. C., 20 minutes, an increase in the stability of the photosensitive material is obtained due to the strong adsorption of the dye onto the silver halide and an increase in the performance of the photosensitive material such as the reciprocity characteristic is obtained due to an improvement in the chemical ripening process upon the addition of the dye.
Moreover, they have discovered that the .phi..sub.r is markedly improved and photosensitive materials of a high sensitivity are obtained by adding heterocyclic compounds having mercapto groups to the emulsion prior to the addition of the dye under the conditions described above.